The effect of free convection on laminar forced flow heat transfer in a horizontal uniformly heated tube was investigated. The Grashof number was varied from 1000 down to the order of one, the low Grashof number runs agreeing with the prediction for pure forced convection within ±8 percent. The effect of the secondary flow created by free convection was observed as a decrease in the wall to bulk fluid temperature difference required to transfer heat as compared to the pure forced flow case in the region far from the tube inlet. This effect was found to increase as the ratio of Grashof number to Reynolds numbers increased. No appreciable effect of free convection was found in the thermal development region.

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